Geochronology and palaeomagnetism of the Hunnedalen dykes, SW Norway: implications for the Sveconorwegian apparent polar wander loop

The post-Sveconorwegian Hunnedalen dyke swarm intrudes the high-grade Proterozoic gneiss complex of SW Norway. The dykes yield a Sm–Nd whole-rock-mineral date of 855±59 Ma and a 40Ar/39Ar biotite plateau date of 848±27 Ma (2σ). The consistency of these two age determinations suggests that 850 Ma approximates the intrusion age of the swarm. Palaeomagnetic data from the dykes match palaeomagnetic directions obtained from Late Sveconorwegian massive-type anorthosites (932–929 Ma) in SW Norway. The collective palaeomagnetic data (mean pole: latitude = 43.7°S, longitude = 213.3°E, A95=4.6°) have uniform magnetic polarity, and, when considered in the context of the new geochronologic data, require that remanence acquisition in Sveconorwegian rocks (massif-type anorthosites and gneissic basement) of SW Norway is, in fact, not Sveconorwegian in age, but rather younger and related to a regional metamorphic/hydrothermal feature at ca. 850 Ma. The new age data question the validity of a counter-clockwise Sveconorwegian apparent polar wander loop in the Baltic data-sets and therefore require a critical re-evaluation of Rodinia Supercontinent fits based on palaeomagnetic data.